Analysis On Seismic Response Of Cooling Tower Considering Subsoil-Foundation-Superstructure Interaction

With the enlargement of power construction scale, the safety requirements of super large cooling tower of large height and diameter are more and more high during the earthquake occurred. It has become an important subject to ensure reliable operation of cooling tower in the seismic.Study on seismic design for cooling tower has the significance of reality and necessary.Subsoil, foundation, superstructure are an organic whole which connect and influence each other.The rigid ground assumption has been used for earthquake response analysis in the traditional seisimic design in which only superstructure is considered in the model and the free field surface input of ground motion is adopted.The rigid ground assumption is advisable,when the foundation has a huge rigidity and the superstructure has a small rigidity.But when the superstructure's rigidity is close to the foundation,this assumption will produces significant errors in determining the superstructure's response.For the super large cooling tower has the character of huge weight and enormous volume,if it is built on the soft soil or deep soil foundation and depended on the rigid ground assumption for seisimic design,without regard to the interaction of subsoil, foundation and superstructure,the result will cause a big discrepancy between calculation and reality. Attention should be paid to effect of subsoil-foundation-superstructure interaction for this kind of building.This paper has summed up the research background, history actuality, analytic method and also existent problems of soil-structure dynamic interaction.It's regarding one real project-the super large cooling tower of thermal power plant as the research example, and building two finite element models in rigid ground and subsoil-foundation-superstructure interaction situation by the platform that large common finite element software ANSYS.Dynamic characteristics analysis and earthquake response analysis under different seismic motion have been made for two models respectively.The influence after interaction consideration is discussed through the comparision of results of dynamic characteristics and dynamic response for two models.Study on effects of dynamic response under near-fault earthquake motion have been made according to the interaction model.The primary content and conclusion of this article are as follows:(1)Research present situation on soil-structure dynamic interaction problem in recent years are genealogical reviewed.Series problems such as method of dynamic analysis, constitutive relation of soil and structure, approximate method of infinite foundation are summarized and concluded.(2)Taking cooling tower of thermal power plant in this paper as the research example,It has been selected the appropriate the material that suited this paper to carry on the research on dynamic interaction.The superstructure has used the model of elastical constitutive relation,the pile-group foundation has quoted the soil-pile equivalent compound model.Rigid ground finite element model and subsoil-foundation-superstructure interaction model are built up respectively by large common finite element software ANSYS in the paper.(3) Dynamic characteristics analysis has been made for rigid ground finite element model and subsoil-foundation-superstructure interaction model respectively.Vibration modes of cooling tower structure under two models show that local vibration modes that lie forward of modes and have great quantity are intensive, overall vibration modes that lie rearward of modes are dispersed.The change of structure dynamic characteristics is discussed in interaction situation,that the natural period is prolonged and the vibration modes is changed.Overall vibration order is greatly advance as compared rigid foundation model.(4) In view of the rigid ground model for cooling tower, The same two earthquake waves have been selected, and a horizontal acceleration time history has been inputted in model for seismic response analysis.The change law of displacements and acceleration was obtained, that the maximal and horizontal displacement of X pillar increases gradually along the height of the tower, and displacement of tower drum decreases gradually. The change law of acceleration amplification factor is the same as the change law of displacements.Comparative analysis has been made for response of structure under different seismic motion; results comparison has manifested effect of different spectrum characteristics of earthquake wave.(5) Aiming at the interaction model for cooling tower, two earthquake waves also have been selected, a horizontal acceleration time history has been inputted in model for seismic response analysis.The change law of displacements and acceleration was obtained, that the maximal and horizontal displacement of X pillar increases gradually along the height of the tower, and displacement of tower drum increases gradually. The change law of acceleration amplification factor is the same as the change law of displacements. Comparative analysis has been made for response of structure under different seismic motion.The paper emphatically analyzes the difference in earthquake reaction of structure in rigid ground and interaction situation,it has been made the summary and the induction to the result regularity,and thought that the influence of interaction does not allow to neglect for structure under the seismic effect.(6) Seismic response analysis has been made for interaction model for cooling tower under the near-fault earthquake motion.Through the contrast of reaction results under different types seismic motion, It showed that seismic response of structure under the near-fault earthquake motion are obviously higher than seismic response of the general ground motion, indicating that this kind of structures are sensitive under the near-fault earthquake motion effect.